Multiple unit control system



4 Sheets-Sheet 1 Oct. 27, 1953 E. s. BRiSTOL MULTIPLE UNIT CONTROL SYSTEM Filed Feb. 14, 1952 8 Km M H s M V mm m w M D E |H I I l II v: l l l I l I 1| B m an an i 5 I O e O? v U9 6 N l no. 99 IIIII o. Q n t I|| E omul ww u Q Q Oct. 27, 1953 E, s. BRISTOL MULTIPLE UNIT CONTROL SYSTEM 4 Sheets-Sheet 2 Filed Feb. 14, 1952 IOSQ 95 v INVENTOR. EDWARD S. BRISTOL ATTORNEYS Oct. 27, 1953 E. s. BRISTOL MULTIPLE UNIT CONTROL SYSTEM 4 Sheets-Sheet 3 Filed Feb. 14, 1952 INVENTOR. EDWARD $.BRISTOL ATTORNEYS Oct. 27, 1953 E; s. BRISTOL 2,657,347

MULTIPLE UNIT CONTROL SYSTEM Filed Feb. 14, 1952 4 sheets-sheet 4 min 44 Z &

rrlm

.0 0| J A (D a N w m 0 w g 5 8% E g M a 2 T fig f INVENTOR. EDWARD S. BRISTOL BY ATTORNEYS Patented Oct. 27, 1953 UNITED STATES PATENT OFFICE MULTIPLE UNIT CONTROL SYSTEM Edward S; Bristol, Philadelphia, Pa., assignor to Leeds and Northrup Company, Philadelphia, Pa., a corporation of Pennsylvania Application February 14, 1952, Serial No. 271,487

I 20 Claims.

This invention relatesto control systems of the type applicable to the control of a plurality of units' which are arranged" to cooperate one with another in a predetermined manner andhas for an object the provision of a control system in which several final control units may jointly respond to a change in-the' magnitude of a condition in accordance with a predetermined schedule of load division between the unit's.

Though the invention has many applications where similar problems of control are involved, it has been foundrparticularly useful in the operation of vapor generators} such for example as the type utilized as a source of steam for steam turbines and the like; As is well understood by those skilled in the: art,.it' is: important to provide pressure-control of the vapor output in order to maintain the generation of vapor or steam in accordance with the load demand imposedupon the turbine. As this load-*de'man'd'rises, the total rate of fuel supply to th'e vapor generator is increased; and viceversa. Inthe modern boiler or vapor generator'a plurality of- 'burners-are utilized, and the totalnee'd'e'd fuel supply must be divided between: the severalburners to secure desired. combustion conditions and adequate distribution of heat within the furnace of the steam generator; For example; it m'ay-be 'de'sire'cl to increase the rate: of fuel supply to certain burners, such as an'upper set' o'f burners; to in'-' crease the'temperature of? combustion gases eXitingv from the furnace and thereby increase the temperature of combustion or flue gas flowing through a'superh'eater section of the steam gen-- erator.

It is a furtherobje'ct of the invention to provide a control. systemiin:which the total fuel demand upon :the vapor generator or boiler is" divided be tween the several burners; with simultaneous change in fuel. supply: thereto in response to change' invapor'd'emand andwith supplementary control for maintaining at selected burners a rate of fuel supply" thereto representing prede termine'dfractions'of'the total load, with the totalfuel supply. varied to meet cha'ng'edconditionsof load demand upon the boiler.

It willbe further understood by those skilled in the art that the heat distribution varies greatly in boilersdepending upon design configuration thereof. and that a predeterminedfuel division between a plurality of -burners-in-a boiler of one type willproducequite different end-re sults as regards combustiona-n'd furnace condi tions, such asslag formation; in -other: types of boilerinstallations. In accordance-with thepres- 2 ent invention, however, the control system pro: vides the needed flexibility to achieve a desired division of the fuelas-between the multipl'e' burnersin any manner needed to establish desired furnace conditions in the boiler.

In carrying out the invention in one for-m thereof, there is provided a-reference circuit 'having adjustable elements which developin the dircuit electrical quantities whose magnitudes are fractions of the total quantity as" determined by the relative adjustments of the" adjustable elements'. A responsive circuit" associated with the reference circuit also includes a mutant-y of ad'- justable control elements to establish" in the re sponsive circuit electrical quantities whose magnitudes are fractions ofthe total quantity and respectively of size dependent uponthe relative adjustments of a plurality of adjustable final control units. More specifica'lly, thefin'al control units to be controlled, the fuel sup ly means-include means operable in accordance with the" ad justment of the fuel supply means to adjust a control element toest'ablish in the responsive circuit an electrical quantity, such as a potential difference. The magnitude of this potential difference is related to the adjustment of the cor: responding final control unit. The potential difference thereby developed is, then, a fractional part of the total potential developed across all ofthc control elements in the respons'ive'cir'c'uit and these fractionalparts correspond to fractions of a total manipulated variable (rate of fuel feed') controlled by'the" final control units; By provid ing detectors responsive to differences in the electrical quantities developed in the respective reference circuit and responsive circuit, an" adjustment of a supplementary nature is provided in accordance with-the settings of the adjustable elements-of the reference circuit. Thus", thetotal fuel demand canbe divided among the several burners as determined by the setting of the ele-' ments' in the reference circuit, while maintaining the total fuel supply to all'of the burners-as'requiredto meet-the steam demand on the vaporgenerator.

With the foregoing outlineof the broadaspects of the invention in mind, it will: be understood that many modifications may be made'and that many further objectsand advantages result therefrom. These will all'be's'et' forth in'detail in the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 diagrammatically illustrates the invention as applied to control the rate of fuel supply t'o-a vapor generator, shown asa-steam'boiler;

Fig. 2 diagrammatically illustrates additional features of the invention as applied to a commercial form thereof to control division of the rate of fuel supply to a steam generator;

Fig. 3 is a schematic representation of another circuit useful in the arrangements of Figs. 1 and 2 and illustrating alternative arrangements for developing potential difference in accordance with the present invention, and

Fig. 4 is a schematic representation of a further circuit embodying the invention and particularly illustrates a control circuit employing current division in the branches of said circuit.

As shown in Fig. l, the invention has been illustrated as applied to the control of the fuel supply means of a steam boiler Ill. The fuel supply means comprises a plurality of burners disposed at different levels in the furnace Illa. At the uppermost level there may be four burners equally spaced around the furnace, as at the corners, two of which, the burners Ila and Ilb, appear in Fig. 1.

As an example of one of the ways in which division of total fuel supply may be utilized, it

is to be noted that the burners lid and III) are nearest the steam super-heater section Illb. Thus, if the total supply of fuel to all of the burners Ila, lib to lie, l lb is maintained the same and a greater fraction of the total fuel is supplied to burners Ila and lib, the volume of the combustion gases passing through steam super-heater section lllb will be the same, but their temperature is elevated to a greater degree than would be the case if the same fractional increase in supply of fuel were made to the lowermost burners I la and l lb. This is due to the greater absorption of heat from the hot combustion gases by the water tubes Illf around the periphery of furnace Illa, in the latter case as compared to the former. that by differently regulating the supply of fuel as between the several burners a substantial measure of control of the temperature of the steam in the super-heater section lllb may be achieved. Further control of that temperature is afforded by the provision of a by-pass section lllc through which combustion gases may flow in avoidance of the steam super-heater section lllb, the volume of combustion gases passing through the by-pass Illc being under the control of any suitable means such as a damper or flowcontrolling element llld, the position of which is adjusted by a motor I5. The motor I5 may be controlled in response to temperature of the steam within the steam output line by any desired system, such as indicated at it under the control of a thermocouple Ill disposed in the steam output line its. Such a system may be of the type fully disclosed in my co-pending application Serial No. 266,818, filed January 1'7, 1952.

In accordance with the embodiment of the in vention of Fig. 1, the load demand upon the vapor generator may be detected by changes in the pressure within the steam header I8 connected to output line lile of the steam boiler I0, which is taken as the controlled variable. As shown, that steam pressure as appearing at header I8 is applied to a master controller device l9, and more particularly, to a Bour-don tube til biased by spring 22 and cperatively engaging a lever 2E. The lever 2i pivoted at 2d positions a valve-actuating element and an air-inlet valve 25 to regulate the admission of fluid under pressure, such as compressed air supplied through Thus, it will be seen i n O a a supply pipe 25. By means or a bleed or leakvalve 21 cooperating with valve 25 the air pressure within chamber 28 and applied to line 29 will have a magnitude dependent upon the steam load on boiler [0, as represented by the steam pressure within header I8. The master controller l9 includes a component of control generally known as reset action provided by a float 30 attached to element 23. Float 30 acts to apply a force to element 23 dependent upon the level of liquid, such as mercury 3|, within a U-tube 32, the application of pressure upon the right-hand leg thereof being under the control or a throttling valve 33.

The pneumatic pressure in line 29 is applied by means of a diaphragm or other pneumatic actuating device 3 through a rod to one side of an arm 36 pivoted at 57. The force applied by the rod 35 is opposed by the magnetic repul sion force developed between a pair of coils 33 and 39 on one side of pivot 31 and the magnetic attractive effort between magnetic coils it and ll disposed on the opposite side of the fulcrum or pivotal mounting 31.

Inasmuch as the steam pressure in the header I8 is an index of the balance between heat input and steam output at boiler ill, the rate or fuel supply or the rate of firing of the burners may, in accordance with the embodiment of the invention shown in Fig. 1, be taken as the manipulated variable. The manipulated variable, the rate of fuel supplied, is the quantity, in Fig. 1, which is varied automatically to maintain the value of the controlled variable, the pressure of the output steam, at the set point, i. e. the pressure selected for preferred operation of the boiler l6. This predetermined pressure is established by adjustment of spring 22 as by hnob 22a, thus establishing the pressure in header I8 which must be developed in the Bourdon tube 29. If the pressure in Bourdon tube 20 rises, the valve 25 closes and leak-valve 21 opens to greater degree and the pressure within pipe 29 decreases. As will later be explained, the action is to decrease the rate of firing to reduce the pressure within header l8 to the set value as established by the setting of spring 22. Upon decrease in steam pressure at header l8, master controller I9 acts to increase the air pressure in pipe 29 and thereby increase the rate of firing to boiler III to return the steam pressure to the set point.

The burners Ila, llb, I201, I21), I311, I32) and Ma, Mb, though of any suitable type, such as oilfired or gas-fired burners, have been illustrated as particularly adapted for the firing of powdered coal. For example, the burners Ila and lib receive coal from a hopper 42 at a rate dependent upon the speed of rotation of a coal-feeding device 43 driven by a variable-speed motor 44, the speed of which is determined as by a resistor 45. Goal at rate determined by the rotation of motor 44 and fuel-feeder 43 is delivered to a pulverizing device 46 which includes a fan or exhauster for producing a flow of air to transport pulverized coal through line ll in suitable condition for immediate burning upon discharge from burner I la. Branch pipes, such as the branch pipe 41a, are provided for supplying other burners, such as burner llb. Similar elements are provided for supplying fuel to each of the remaining burners, the coal-feeding devices 46, t3 and 55 being operated by motors 5l, 52 and 53, respectively provided with speed-regulating means, such as series resistors 54, 55 and 56. It will be observed that the pulverizing devices 5?, 53 and 59 associated with feeders 48, 49 and function in the same manner as the device 46, and it will be further observed that the branch 60a. from pipe 60 is shown as extending to the lowermost burner [41), in manner above described.

From the foregoing it will be seen that the fuel feeders represent adjustable final control units, the adjustment or speed of operation of which determines the amount of fuel delivered to its associated burners for steam generator it.

Means are provided to measure the total amount of fuel delivered to the boiler, such means in Fig. 1 being illustrated as tachometers 6I54. Since the output rate of pulverizing devices 46, 5l-59 is substantially proportional to the fuel input rate, the rate of fuel supply in the form of pulverized or powdered coal depends upon the speed of operation of each of the control units 43, 48-50. Accordingly, the voltage developed by each of the tachometer generators 5l-64 driven by motors 44, 5l 53, will be representative of the rate of feed of fuel of each corresponding unit. By electrically connecting the tachometer generators in a series totalizing circuit the total rate of firing or fuel supply will be represented by the sum of the voltages of the individual tachometers. This voltage is applied by way of a calibrating resistor 65 to the seriesconnected coils 38-4! of a fuel feed controller liii. Thus, the fuel feed controller 66 operates in response to unbalanced forces as between those developed by the coils 384i and the pressure applied from the pneumatic actuator 34.

When the force applied by the actuator 34 is greater than that developed by the coils, resulting from a decrease in steam pressure at header Hi, the arm 35 is tilted about its fulcrum 3? first to complete a circuit between a movable contact 61 supported by arm 36 and a stationary contact 68 and, second, to complete an energizing circuit from one side 69a of a source of supply for relay coil 70 of a switch actuator 72 and thence to the other side 59b of the supply source. Under the control of the switch actuator 12 each of motors 44, 5l53 is adjusted to increase the rate of fuel supply and preferably by equal amounts. This change in rate of fuel will result in returning the steam pressure at the header Hi to the set point.

In more detail, when the coil I0 is energized, an arm l3, shown biased to mid-position by a pair of springs, is rotated in'clockwise direction to actuate movable contacts '54-'41 to complete circuits from one supply line tSa through upper stationary contacts for energization of the increase, I, held windings of motors lile3l. By energizing these windings, simultaneous rotation of the motors is produced in a direction to move contacts a, 54a, 55a and 56a to decrease the resistance of speed-controlling resistors in circuit with each of motors 44, 5l-ii3 and thereby increase the speed thereof.

It will be observed that the energizing circuit for the coil from line 69a includes a circuit interrupter 82. The interrupter B2 is periodically opened and closed by any suitable means, such as a cam 83 driven by a constant speed or synchronous motor 84. The effect of the cyclically operable interrupter 32 is to provide current pulses through coil ill which actuates the movable contacts 'l4l'l into and out of engagement with their upper contacts to provide speed changes of motors 44, SI-bt by a series of steps. By interposing the step adjustment of the speed of the motors, a certain amount of time is provided for the reflection of change in pressure in header l8 as a result of a change in rate of fuel supplied to the steam generator [0. However, if the pressure in header It be substantially less than desired, the increased pressure in line 29 actuates the movable contact 61 sufficiently to displace upper contact $8 and to bring lower contact 85 (insulated from contact 67) into engagement with stationary contact 86. There is thereby completed by way of conductor 37 a by-pass circuit around the interrupter contact 82 for con tinuous energization of coil 10 and continuous completion of the motor circuits l88l for continuous adjustment of the speed of fuel-feeding devices 43, 43-50 until the voltage generated by the tachometers fil$4- produces an opposing force which first opens contacts 85, 86 and subsequently opens contacts El, 68, there being a transition from continuous operation of the motors 13-8! to intermittent operation as above described.

Upon an increase in the pressure in header l8 reversed operation occurs which will not be described in the same detail, it being merely necessary to state that movable contact 61 is moved into engagement with a stationary contact 88 to complete an energizing circuit for the actuating coil ll of actuator 12, this circuit being traced by way of the intermittently actuated interrupter 82. When the deviation is beyond a predetermined amount, movable contact 85 is moved into engagement with stationary contact 89 to complete a by-pass circuit by way of conductor 8! around the interrupter for continuous energize.- tion of coil H. The movement of actuator l2 by coil H moves contacts 74-11 into engagement with the lower stationary contacts for energization of the decrease windings D of motors EB-23! for rotation thereof in the opposite direction to adjust contacts 45a, 54a, 55a and 56a to increase the resistance in series with the associated motors and thereby decrease the speed of operation of the fuel-feeding devices 43, 48-430.

With the understanding of the several parts and the manner in which they cooperate together, reference will now be had to the manner in which an additional control means functions to modify the action so that the final control units 43 and 48-439 may be readjusted to divide the rate of fuel supply between them in such manner as may be desired without changing the total rate of fuel delivery required, as determined by the master control means. The master control means may be defined to include the master controller l8, the fuel-feed controller 66, the switches i k-ll, their actuator 12, the motors '|8Bl, the speed ad- Justers and their motors 44, M e-3, as well as tachometers 6|-64,

The additional control means includes an electrical network 90 which in one form of the invention includes a responsive branch having a plurality of slidewires or variable resistors corresponding with each of the final control units 43. and identified respectively as slidewires ill-Q4. In a second or reference branch of the network there are provided additional variable resistors or slidewires 95, 9?, 99, It! and in series therewith fixed resistors 96, at, I!!!) and H12. It will be understood that the fixed resistors are not essential but are preferred to limit the effect of the additional control means.

The two branches of the circuit are connected to a suitable source of voltage as represented by supply lines I65 and H16. The movable contacts 9Iall4a associated with slidewires 9|94 are respectively driven by motors EB-4H. The voltage between lines H15 and [BB divides across slidewires 9l-94. If the resistance values of each slidewire be the same, it is important to observe that the voltage division will be equal; that is to say, as long as the movable contacts 9la 94a are moved by equal amounts similarly to change the resistance of slidewires 9l-94, the potential differences developed in the responsive branch of the circuit will remain the same fractional parts of the total voltage between lines 185 and I06, irrespective of the value of that total voltage.

It will be understood that the voltage across the resistors in the other branch of the circuit will divide in like manner. More particularly, as long as the resistance value of a slidewire, such as slidewire 95 and its associated resistor 96 are equal to the resistance value of each remaining slidewire and resistor, the voltage will equally divide or the potential differences developed by them in that branch of the circuit will be of equal magnitude one to another and will each be the same fractional part of the total voltage between lines I and H16, irrespective of the absolute value of the total.

Advantage is taken of the above characteristics of the network 90 as a part of an additional means to re-adjust the final control units 43, 4850 to divide the total rate of fuel supply (the magnitude of the manipulated variable) amongst the several control units in any desired manner without changing the total magnitude established by the master control means as required to maintain the pressure within header I8 at the control or set point. The foregoing re-adjustment of the control units is accom plished by manual adjustment of slidewires 95, 91, 99 and IN, as by their associated knobs. The movable contacts associated with each of these slidewires is illustrated in the mid-posh tion.

If the effective resistance value of one of the slidewires 95, 91, 99, or IUI is increased, in what has been referred to as the reference branch of the circuit, the voltage division across the several parts thereof is changed. More specifically, if the resistance of slidewire 95 is increased, the potential difference across slidewire 95 and its associated resistor 96 is greater by the amount of the increased resistance than the potential difference across slidewire 91 and its associated resistor 98, the same also being true for slidewire 99, its associated resistor I80, and for slidewire I0! and its associated resistor I02. Accordingly, a potential appears between the points A and B to which a detector I91, including an amplifier Him, is connected. There will be applied to the input circuit of the detector a potential difference with the point A at a higher potential than that of point B. In response to this supplementary signal, a relay l 01?) of detector In! is energized to complete a circuit through the upper stationary contact for the winding I of motor 18 for energization of the winding in a direction to increase the speed of motor 44, tachometer BI and fuel-feeding device 43.

The energizing circuit may be traced from supply line 69a, the periodically-operated switch H0, conductors HI and H2, the upper stationary contact, motor winding I, motor armature, and thence to the other supply conductor 69b. The switch H9 is periodically opened and closed by any suitable means such as a cam H3 driven by a constant speed or synchronous motor H4. The motors 84 and H4 are operated in predetermined phase relationship one with the other, and as shown, switch 82 is closed while switch III] is open. The phasing and the relative periods during which the switches 82 and H0 are operated and remain closed may be widely varied. There is nothing of a critical aspect to the adjustments, the preferred arrangement having been illustrated and described. Both control actions may be continuous, if desired.

It will be understood that the adjustment of the motor 18 occurs during the time interval of closure of switch H0. With rotation of motor 18 the movable contact of slidewire 9| is adjusted in a direction to increase the resistance in the responsive branch of network 90, this being in the correct direction to decrease the potential difference between points A and B. By reason of the increase in speed of tachometer 6|, the force exerted by coils 38--4l on pivoted lever 36 is increased to complete through its contacts 88 or 89 an energizing circuit for the coil H for operation of movable contacts 14--Tf into engagement with their lower stationary contacts for energization of motors 18-8! in a direction to decrease the speed of motors 44, 5l--53, their associated tachometer generators 6|, 62, 63, 64 and fuel-feeding devices 43, 48-50. The speed of motor 44, of tachometer GI and fuel-feeding device 43 remains relatively greater than that of motors 5l--53, and the effect of the decreased speed of the motors 5I--53 is in a direction to compensate for the increased speed of motor 44. Stated differently, the tachometer circuit is a totalizing circuit representative of total manipulated variable rate of fuel delivered to the steam generator l9. Thus, with increased speed of motor 44 and fuel-feeding device 43, there is produced a compensating adjustment of motors 51-53 and fuel-feeding devices 4850 which maintains the total fuel delivered to the boiler at the same rate as before.

There is some interaction and cooperation between detectors I08 and I09. However, in practice after a manual adjustment has been made of one of slidewires 95, 91, 99 or fill, the system after a short interval of time readjusts the speeds of motors 44 and 5l-53 to deliver to the steam generator I 0 fuel at selected rates through the several banks of burners with the same total fuel delivery as before adjustments of said slidewires.

From the foregoing it will be seen that with but three detectors HIT-I09 the four fuel-feeding devices 43, 48-50 can be controlled to operate at different selected speeds without change of total fuel delivery to the steam generator. In the foregoing description the assumption was tacitly made that the load demand on the boiler ID has remained constant and that a constant force has been applied to the lever 36 by the rod 35 from the pneumatic actuator 34.

If the slidewire had been adjusted in the opposite direction, that is, to decrease the value of the resistance in the reference circuit, the new division of voltage resulting therefrom produces a potential difference between points A and B of polarity opposite to that described above. Thus, the motor winding 1) of motor 13 is energized through detector 10? and energizing relay lfllb to complete an energizing circuit therefor from supply line 69a through the lower stationary contact of relay Illl'b. Motor 18 decreases the speed of motor 44 and the tachometer 6|. The decrease in the voltage at tachometer 6|. results in rotation of pivoted lever 36 in the opposite direction to complete a circuit through the coil iii of actuator l2; Through the movable contacts- M-'II the motors IB-8I are energized in directions to increase their respective speeds, thus restoring voltage balance in what has been termed the totalizing circuit including the tachometers. Thus, a decrease in speed of motor 44 results in an increase in the speeds of motors E'I-ES by amounts which restore the total. fuel-delivcry to the steam boiler It to the same value it had before the adjustment or" slidewire95.

Though a detector is not provided for auxiliary control of motor SI, nevertheless the slidewire 94 will be adjusted by motor 8! to bring to zero value the potential between points and F and to bring the slidewire {it to a position corresponding with that of slidewire I iI. More particularly, if slidewire Ifl'I be adjusted to increase its resistance in the reference circuit, again assuming that the load demand is constant and that the steam pressure is at the control, point, there will be immediately changed the voltage distribution across the resistance elements in the reference branch, and potential differences will appear between points E and F, C and D, andA and B. The detectors Iil'II99 will each respond to their respective potential diiferences and will energize motors IS-8i] in.

94a is in a direction to increase the resistancevalue of each slidewire Gil-94 in the responsivebranch of the circuit and that the potential difference which appeared between points E and" F solely by reason of adjustment of slidewire IdI is reduced and it eventually brought to zero value by the combined operation of the master control means and the additional con-- trol means. At termination of the combined control actions, the end result is that slidewires 9I-93 occupy positions to reduce to zero the.

Voltage between points A and B, C and D, and E and F, while slidewire 9c is brought, to a position of increased resistance where the voltagedeveloped or the potential difference across. slidewire 94 is equal to the potential difference,

developed across slidewire [SI and resistor I02. Thus, there will have been accomplished areadjustment of the speed of motor 53 inaccord-- ance with the setting of slidewire resistor [III with corresponding compensating adjustments in the speeds of motors 44, and 52 to maintain substantially constant the total rate of fuel delivered to boiler Ill.

It is to be understood that the master con.-

troller in cooperation with the fuel-feed. con-- troller 66 functions through the. upper contacts 68 and 88 and the, lower contacts 8 and 89 for control of the actuator I2 to readjust the speeds of motors 44; 5.i5i3. to change the total fuel. delivered to the boiler If! in accordance with. theload demand. However, there isalways superimposed upon the-control of total fuel de- It Will be Cil 10 livery, the supplementary control which divides the fuel requirements between the several fuelfeeding devices in manner determined by the settings or adjustments of the slidewires 9?, 99 and Illi.

While the system of Fig. i has spects included diagrammatic rep son" tions of the several elements, it is again emphasized that many variations in components may be .iade to adapt the invention to applications of widely differing character.

In Fig. 2 there has been illustrated additional features ofthe invention utilized in connection with a commercial form of the apparatus. In this arrangement, the network has been illustrated as being supplied by alternating current, from any suitable source by way of power transformer Iat through a swamping resistor I'2I to conductors Iii-5a and I Etc. The detectors I 08 and I89 have been illustrated in the same manner as in Fig. 1, but the detector I ill has been illustrated in detail. It will be understood that detectors I88 and Its are of the same type as I81. and that each comprises an input transformer 522, the primary winding of which is connected across the input points, such as A and B, of network 933. The outer ends of the secondary winding of transformer I22 are connected to the grids of thermionic tubes I23 and i2 3, while their cathodes are effectively connected. to the mid point of the secondary winding of transformer I22 as by resistors I 25 and I26. It will be observed that the cathode-grid. circuits include a fractional part of the secondary winding of a transformer IZ'I. supplied.

from a suitable alternating-current source. The connection is such that when the anodes or. plates of tubes I23 or I24 are positive with respect to cathode, the grids of the two tubes are made adequately negative with respect to.

tov the plate-cathode circuits of tubes I23 and I24 by transformer it? will depend upon the.

If the podirection of the voltage unbalance. tential. difference or voltage division across slidewire BI is less than the potential difference across slidewire 95 and its associated resistor 95, the tube I23 will be made conductive, while the tube I24 will, to even greater degree, be rendered nonconductive. Thus, current will flow from one side of the secondary winding of transformer I21 by way of relay winding itl'z' through the tube I23" and by way of conductor I28 to the other side of the transformer. Since only half cycles of. one polarity flow through coil IIl'Iz, a. capacitor I3!) is provided in shunt with the coil to maintain it energized during the period the tube is not conductive, corresponding with the appearance of negative half-cycles of voltage across the secondary winding of, transformer E21. When the potential unbalance is in the opposite direction with a greater potential difference across slidewire 95, the phase of the signal applied to the inputtransformer reverses, tube 524 is rendered conductive, and relay Itld is energized with acapacitor I3! maintaining the coil energized during the negative half-cycle in manner already described.

Assuming now that the relay coil I012 is energized as above described, the energizing circuit for the operating coil of a motor contactor is partially completed by the closure of relay contacts Itlc. This circuit is completed as soon as the synchronous motor I33 rotates cam I34 so that the crest thereof closes the contacts I35. The energizing circuit may then be traced from line LI through contacts I35, contacts Ifllc, operating coil of contactor I322, contacts I36a of a limit switch L36, and thence to the other supply line I.--2. Accordingly, the contactor I321 closes and energizes a motor I31, shown as bein of the three-phase induction type, to rotate a speed-controlling member of a variable speed drive [46. As schematically illustrated, the speed-controlling drive I48 includes threaded member MI which adjusts the driving element between a pair of conical members I42 and I43, the former being driven by any suitable means, such as motor I44. The conical element I43 drives the fuel-feeding device 43 at a speed determined by the setting of the driving element positioned by threaded member I41.

It will be observed that in Fig. 2 the coal from hopper 42 is fed by unit 43 to a pulverizing mill I46 of pulverizing unit 46. After reduction of particle size the coal is delivered by an exhauster fan I41 and is forced through fuel supply lines 41 and 41a to the burners Ila and III) (Fig. 1). It will be observed the motor I44 by drive shaft I48 drives both the pulverizer I46 and the ex-- hauster fan I41.

Returning now to the operation of the system, it will be recalled that upon energization of motor l3! the relative position between slidewire 9| and its contact Ella is changed, the mechanical connection being shown by the broken line I59. The length of time the motor I3"! is energized is determined in the first instance by the energization of relay NW, and secondly by the movement of cams I34 to close contacts I35.

The operation of the detector I07 and detectors I63 and I69 is the same as in the modification described in Fig. 1. Each of the variable speed drives H40 will have associated with it a tachometer, only one of them being shown, the tachom eter GI together with its associated permanent magnet Ma. The remaining tachometers will be electrically connected in a totalizing circuit in the same manner as shown in Fig. 1, and they will function as there described to apply a force to the pivoted operating lever 36 of the fuel-feed controller 66, a fractional part only of which appears in Fig. 2.

When relay Ill'Id is energized, a similar energizing circuit is completed by way of its contacts IO'Ie for the operating coil of the other motor contactor I32d for energization of the motor I37 in the reverse direction, namely, to decrease the speed of the final control unit or coal feeder 43 to decrease the rate of fuel delivered to the steam generator I6. The motor contactors I322 and I32d in reversing one phase only of the threephase alternating-current source of supply are provided with a mechanical interlock I46 so that after energization of the coil of one of the contactors I321 or I320? the other may not be closed upon energization of its coil.

While separate limit switches may be utilized, the single limit switch I36 has been shown associated with a single cam I 52 driven through mechanical connection I50 so that upon movement 12 of the driving element positioned by screw 'I4I to near its low-speed position, the lever I53 is actuated by the cam to open limit switch contacts 36d and I36e, while at the position of the maximum speed for unit 43, the levers I54 and I55 open contacts I36a and I361.

At times in the operation of a steam boiler it may be desired to control the rate of firing or fuel delivery thereto by manual control of one or more fuel feeders independently of the automatic control already described in detail. As shown in Fig. 2, this is conveniently done by operation of a transfer switch I66 as by raising handle IBI to open contacts I62 and I63 and to close contacts I64 and IE5. The opening of contacts I62 opens the energizing circuits controlled by relays 76, 'II, I611 and IBM. The opening of contacts I63 opens the energizing circuit for the operating coil of a relay 166 which closes its contacts to short-circuit the detector I61. By connecting points A and 13 together the potential therebetween is equalized at the same time that the input of detector I 0? is short-circuited.

The closure of contacts I64 by operation of transfer switch I60 partially completes an energizing circuit for the motor contactor I322 which can be completed at will by operation of a pushbutton switch I6l'. Similarly, the closure of contacts 65 partially completes the energizing circuit for the motor contactor I32d which may be completed at will by the push-button switch I63. Thus, either of motor contactors I321 or I32d can be energized manually for change of speed of the control unit :13 in a desired direction.

It is to be observed that with relay I66 de-energized and the potential equalized as between points A and B, the detectors I68 and I69 may continue to function to control their respective final control units 48 and 49 (Fig. 1) automatically and in cooperation with the fuel-feed controller 66 will position final control units 56 and 5! in manner already described. In this connection the tachometer SI remains in the totalizing circuit in series with the remaining tachometers 62-54 so that the force applied to the pivoted lever 36 by coils 3B-4I is representative of the total fuel feed to the steam generator III. The result is that the automatic control remaining in operation functions to maintain the total rate of fuel delivery at a constant value, notwithstanding the change in fuel delivery effected by Znanual control of the control unit or coal feeder While only one of the detectors I61 and associated control circuits has been illustrated in detail in Fig. 2, it is to be understood that each of the remaining control units 48-56 will be provided with identical control systems and that any one of them may be changed from manuel to automatic operation with the several tachometers continuing to function in the totalizing circuit as above described. The only difference between the remaining control systems is the arrangement of contacts on the relays corresponding with the relay I66. When relay H9 is deenergized, the arrangement of contacts for relay I'IIl is such that contacts Vila short-circuit or reduce to zero the potential drop across slidewire 92 in the responsive circuit and contacts I'Ifib shortcircuit or reduce to zero the potential drop across slidewire 91 and its associated resistor 66 in the reference circuit. By operation of contacts I'Iila and H01) the potential between the points A and C is equalized, and the potential between points B and D is equalized. Similarly, the contacts 13 "id and ill!) of a relay I'll equalize the potentials between points C and E as well as between points D and F.

There is one further variation in the control means associated with the drive of contact 84a and with a relay I72 having contacts N20. for reducing the potential between the points E and F to zero. For reasons explained hereinbefore, there is omitted from such control means the counterpart of relays I071 and lllld and their associated functions, since a detector is not associated with the control for the fuel-feeding unit 50. I

While the invention in both Figs. 1 and 2 has been set forth in connection with the control of four final control units, it is, of course, to be understood thatthe principles involved are applicable to any desired number-two or more-of final control units. In a system with a greater number of units, the control circuits and switching arrangements for the intermediate detectors will correspond with those provided for detectors 108 and I09, while the control circuits at the outermost parts of the control system 90 will correspond with relays I66 and H2. Likewise it is to be noted that the supplementary or additional control means provided by network 90 and detectors ll|09 is capable of functioning independently of or in the absence of the master control means including master controller is, fuel -feed controller 66 and tachometer generators (ii-64 to maintain predetermined relations between the control units. Such a system may be employed where it is not essential to maintain the total fuel supply at a predetermined value.

type as that shown in Figs. 1 and 2, including slidewires 95, 9?, 99 and [0! as well as the associated resistances 96, 98, I and I02. However, in the responsive branch of network 00a, potential differences are developed across impedance members 200, 2-01, 202 and 203 which in the pres- 5 ent arrangement are secondary windings of transformers 204-201. The potential differences across windings 200-203 are of course dependent upon the voltages applied to the primaries of transformers 204-201. These primary windings are respectively arranged to be energized from variable output auto-transformers 208-21 I. The variable auto-transformers 208-211 are excited from a source of alternating current potential such as lines 212, 2|3. Associated with each of the variable output auto-transformers 2il8-2'H are movable contacts 208a-2ll'a respectively moved in accordance with the rotation of motors l88"l. As shown in both Fig. 3 and Fig. l, the the rotation of motors T6'8| controls the positioning of contacts a and Eta-56a on resistors 45, 54-56 respectively to control the speed of motors '44 and 5I-53 and the final control units 43, 48-50.

In operation, the potentials developed across impedance elements 200-203 function in manner similar to the arrangements shown in Figs. 1 and 2, since elements 200-203 establish a potential division in the responsive branch of network 90a, in the same manner that slidewires 91-04 perform that function in Figs. 1 and 2. However, in the present arrangement impedance elements 200-203 are also potential developing means and the total potential developed between lines [051) and |06b will be the sum of the individual potentials across windings 200-203.

A particular advantage of the arrangement of Fig. 3 is that the totalizing circuit including coils 38-4! of fuel-feed controller 66 is supplied directly from network a rather than from a separate circuit requiring auxiliary potential generating devices such as tachometer generators til-64, as in Figs. 1 and 2. In the present embodiment the totalizing circuit is energized by means of a full-wave rectifier 210 connected across lines Hi5!) and [0612 through a variable resistor H5 and a supplementary transformer M6. The transformer 21 6, whose primary may be supplied by any suitable source of alternating current provides compensation for the variable resistance characteristics of the rectifier elements of full-wave rectifier 2M. In practice, the potential applied by the secondary winding of transformer 2 l 6 may be from 4 to 6 volts as compared to a total voltage between lines i051) and 10619 of from 80 to 100 Volts for maximum fuel supply by final control units 03, 48-50. The outpue of rectifier 2M is connected in series with coils 38-4! of fuel-feed controller 66 by way of lines 2H and 2H3.

From the foregoing description of the arrangement of Fig. 3, it will be apparent that upon adjustment of the slidewires 95, 91, 99 or l0l, detectors l0'i-ll39 respectively operate to drive motors '18-'30 in manner described in connection with Figs. 1 and 2, which motors, as well as motor 8|, are arranged to drive movable contacts 2080-2Ha. Movement of contacts 209a- 2lla varies the potential applied from autotransformers to transformers 204-201 so as to maintain the potential difference between points A and B, C and D, as well as E and F at substantially zero in the same manner as described in the operation of Figs. 1 and 2.

While the functions of the slidewires and resistances in the reference circuit are substantially identical with the arrangement of Figs. 1 and 2, desirably, the total resistance across elements -l02 in the present arrangement is substantially higher than the circuit through rectifier 2M and coils 38-4.! of fuel-feed controller 66. By making this total resistance high, adjustment of any of the reference slidewires 05, 91, 99 or l0l will not have appreciable effect upon current flow through coils 38-41.

In the arrangement of Fig. 4, there is shown in schematic form a further modification of the supplementary control circuit, identified in the present embodiment as 901). As particularly distinguished from the arrangements of Figs. 1-3, that of Fig. 4 illustrates the present invention as applied to a control network in which the electrical quantity to be divided in the responsive circuit and reference circuit is a total current now, as distinguished from a total potential difference. The reference branch of control network 901), including slidewire resistors 25a, 25!, 252 and 253 is so connected with the responsive branch of the network, including slidewires 254- 251, that the total current through both branches is the same. In the circuit shown in Fig. 4 both the reference and responsive branches are connected to a source of current through input lines 258 and 259 and the two branches interconnected by conductor 2-60.

As particularly distinguished from the foregoing embodiment, it will be observed that the slidewires 250-253 in the reference branch are connected in parallel, and in series with each of the slidewires 250, and 252 there are respectively connected current detector windings or coils 26!, 262 and 263. Each of the slidewires 250-253 may have connected in series therewith fixed resistances 264-261. Correspondingly, the adjustable control elements 254-257 in the responsive branch of network 891) are connected in parallel; and in series with elements 254, 255 and 256, there are connected current detector windings or coils 268, 269 and 270. The windings 26! and 268 are mechanically arranged to form a differential current detector means for operation of an armature 21! in accordance with the sense of unbalance of current flow in the re spective circuits to which responsive windings 26l and 268 are connected. Armature 2' is arranged to connect drive motor 78 through either its increase, I, winding or its decrease, D, winding in manner similar to the operation of detector relay mil) in Fig. 1. In similar manner, the direction of drive of motors l9 and 80 may be controlled by the movement of armature 212 positioned between windings 2512 and 269 and the direction of drive of motor 80 may be controlled by movement of armature 213 positioned between windings 253 and 210.

In the operation of the arrangement of Fig. 4,. upon any relative change in current flow in one of the four branches of the reference circuit as determined by the relative adjustments of slidewires 250-253, associated current detectors will Operate armatures 2ll-2l3 to establish corresponding relative current fiows in the associated branches of the responsive circuit in manner to make the division of current in each branch of the responsive circuit similar to the current division in each branch of the reference circuit.

Though. the terms used herein in general have the same meaning as the definitions proposed by the Terminology Committee of the Industrial 1nstruments and Regulators division of the American Society of Mechanical Engineers (see Mechanical Engineering for February 1946), they are susceptible of somewhat broader interpretations. For example, the controlled variable is in general a quantity or condition which is measured and controlled while the manipulated variable is the quantity or condition which is Varied by an automatic controller or by the final control element or unit. However, the two variables in some instances may be the same and hence the use of one term is not used to the exclusion of the other. Similarly, the reference to an electrical quantity is to be taken as generic to voltage or current or to another selected electrical condition.

While modifications and changes will become apparent to those skilled in the art from the foregoing description, all such modifications and changes as fall within the scope of the appended claims are intended to be embraced thereby.

What is claimed is:

1. A control system comprising adjustable final control units, control elements connected together in a responsive circuit and respectively corresponding with said control units, means operable in accordance with adjustments of said units for adjusting said control elements to establish in said responsive circuit electrical quantities whose magnitudes are fractions of a total electrical quantity and respectively of fractional size dependent upon the extent of relative adjustments of said units, a reference circuit having elements for establishing electrical quantities whose magnitudes are desired fractions of a total electrical quantity as determined by the relative values of said reference circuit elements one to the other, and means including electrical quantity difference detectors respectively connected between different portions of said circuits for changing the adjustments of said units until the fractional electrical quantities of said responsive circuit and of said reference circuit bear the same relationships one to the other.

2. A control system comprising adjustable final control units, control elements connected together in a responsive circuit and respectively corresponding with said control units, means operable in accordance with adjustments of said units for adjusting said control elements to establish in said responsive circuit potential differences whose magnitudes are fractions of the total potential across said responsive circuit and respectively of fractional size dependent upon the extent of relative adjustment of said units,

a reference circuit having elements for developing potential differences whose magnitudes are desired fractions of the total potential across said reference circuit as determined by the relative values of said reference circuit elements one to the other, and means including detectors respectively connected between different portions of said circuits for changing the adjustment of said units until the fractional potential differences of said responsive circuit and of said reference circuit bear the same relationships one to the other.

3. A control system comprising adjustable final control units, control elements at least one for each control unit connected together in a responsive circuit, means operable in accordance with the adjustments of said units for adjusting said control elements to establish in said responsive circuit electrical currents whose magnitudes are fractions of the total current in said responsive circuit and respectively of fractional size dependent upon the extent of relative adjustments of said units, a reference circuit having elements for establishing electrical currents whose magnitudes are desired fractions of the total current in said reference circuit as determined by the relative values of said reference circuit elements one to the other, and means including detectors respectively connected between different portions of said circuits for changing the adjustments of said units until the fractional currents of said responsive circuit and of said reference circuit bear the same relationships one to the other.

i. A control system comprising adjustable final control units, control elements connected together in a responsive circuit and respectively corresponding with said control units, means operable in accordance with adjustments of said units for adjusting said control elements to establish in said responsive circuit electrical quantities whose magnitudes are fractions of a total electrical quantity and respectively of fractional size dependent upon the extent of relative adjustments of said units, a reference circuit having adjustable elements for establishing electrical quantities whose magnitudes are desired fractions of a total electrical quantity as determined by the relative adjustments of said reference circuit elements one to the other, and

17 means including electrical quantity difference detectors respectively connected between different portions of said circuits for changing the adjustments of said units until the fractional electrical quantities of said responsive circuit and of said reference circuit bear the same reiationships one to the other.

5. A control system comprising adjustable final control units, control elements connected together in a responsive circuit and respectively corresponding with said control units, means oprable in accordance with adjustments of said units for adjusting said control elements to establish in said responsive circuit electrical quantities whose magnitudes are fractions of a total electrical quantity and respectively of fractional size dependent upon the extent of relative adjustments of said units, a reference circuit having adjustable elements for establishing electrical quantities Whose magnitudes are desired fractions of said total electrical quantity as determined by their relative adjustments, one to the other, and means including electrical quantity difference detectors respectively connected between different portions of said circuits for changing the adjustments of all but one of said units until the fractional electrical quantities of said responsive circuit and of said reference circuit the same relationships one to the other.

A control system for maintaining a controlled variable at a set point comprising a plurality of adjustable final control units, each establishing a fractional part of a man pulated variable, control elements connected together in a responsive circuit and respectively associated with said control units, means operable in accordance with adjustments of said units for adjusting said control elements to establish in said responsive circuit electrical quantities whose magnitudes are fractions of a total electrical quantity and respectively of fractional size depending upon the extent of relative adjustments of said units and the total manipulated variable controlled by said units, a reference circuit having adjustable elements for establishing electrical quantities which represent fractions of a total electrical quantity as determined by their relative adjustments, one to the other, for predetermining the division between said units of said manipulated variable, and means including electrical changing the adjustments of said units until the fractional electrical quantities of said responsive circuit and of said reference circuit bear the one to the other thereby establishing the fractional parts of said manipulated variable controlled by each of said units.

'7. A control system for maintaining a controlled variable at a set point comprising a plurality of adjustable final control units, each establishing a fractional part of a manipulated variable, control elements connected together in a responsive circuit and respectively associated with said control units, means operable in ac cordance with adjustments of said units for adjusting said control elements to establish in said responsive circuit electrical quantities whose magnitudes are fractions of a total electrical quantity respectively of fractional size depending upon the extent of relative adjustments of said units and the total manipulated variable controlled by said units, a reference circuit having adjustable elements for establishing electrical quantitieswhich represent fractions of a'total electrical quantity as determined by their relaquantity difference detectors for F tive adjustments, one to the other, for predetermining the division between said units of said total manipulated variable, means including electrical quantity difference detectors for changing the adjustments of said units until the fractional electrical quantities of said responsive circuit and of said reference circuit bear the same relationship one to the other thereby producing said division of said total manipulated variable between said units, and means for simultaneously adjusting all of said units in response to deviation of said manipulated variable from a predetermined value.

8. A control system for maintaining a controlled variable at a set point comprising a plurality of'adju'stable final control units, each establishing a fractional part of a manipulated variable, control elements connected together in a responsive circuit and respectively associated with said control units, means operable in accordance with adjustments-of said units for adjusting said control elements to establish in said responsive circuit electrical quantities whose magnitudes are fractions of a total electrical quantity and respectively of fractional size depending upon the extent of relative adjustments of said units and the total manipulated variable controlled by said units, a reference circuit having adjustable elements for establishing electrical quantities which represent fractions of said-total electrical quantity as determined by their relative adjustments, one tothe other, for predetermining the division between said units of said manipulated variable, means including detectors for changing the adjustments of all but one of said units until the fractional electrical quantities of said responsive circuit and of said reference circuit bear the same relationship one to the other thereby producing said division of said manipulated variable between saidunits, and means for simultaneously adjusting all of said units in response to deviation of said manipulated variable from a predetermined value.

9. A control system for maintaining a controlled variable ata set point comprising a plurality .of-adjustable final control units, each establishing a fractional value of a total manipulated variable, acontrol element for each of said units capable of developing a potential difference, a responsive circuit in which said control el ments are connected, master control means responsive to the magnitude of the controlled variable for simultaneously adjusting said final control units for maintaining that total magnitude of said manipulated variable as required to maintain said controlled variable at said set point with simultaneous related adjustment of said control elements, each of said control elements developing in said responsive circuit a potential difference whose magnitude is a fraction of the total potential across said responsive circuit and of fractional size dependent upon adjustment of its associated unit relative toadjustments of other of said units, means including a reference circuit having adjustable elements for developing potential differences of magnitudes which are fractions of the total potential across said reference circuit as determined by differences one to the other in their relative settings, and additional means to readjust said control units to divide said total magnitude of the manipulated variable between the several control units, said additional means ineluding detectors for changing the adjustment of all but one'ofsaid units in response to differences between the fractional potential differences in aceaccv 19 said responsive circuit and the corresponding fractional potential differences in said reference circuit.

1. A control system for maintaining a controlled variable at a set point comprising a plurality of adjustable final control units, each establishing a fractional value of a total manipulated variable, a control element for each of said units capable of developing a potential difference, a responsive circuit in which said control elements are connected, master control means responsive to the magnitude of the controlled variable for simultaireou adjusting said final control units for maint n 1g that total magnitude of said manipulates. variable as required to maintain said controlled variable at said set point with simultaneous related adjustment of said control elements, a iof said. control elements developing said responsive circuit a potential difference whose magnitude is a fraction of the total potenacross responsive circuit and of fractional size dependent upon adjustment of its associated unit relative to adjustments of other of said units and which fractions are independent of the total po ential difference across said responsive circuit, including a reference circuit having adjustacle elements for developing potential differences oi magnitudes which are fractions of the total potential across said reference circuit as determined by differences one to the other in their relative settings and which fractions are in pendent of said total potential across said re nce circuit, and additional means to readjust said control units to divide said total magnitude of the manipulated variable between the several control units, said additional means including detectors for changing the adjustment or" all but one of said units in r sponse to differences between the fractional potential differences in said responsive circuit and the corresponding fr etional -potential differences in said reference circuit.

ll. A control system for dividing a manipulated variable among a plurality of final control units comprising a plurality of series connected control elements and a plurality of series connected adjus elements, connected in parallel to a source of potential, detector means connected between the junction of two of said control elements and two of said adjustable impedance elements, means operable in response to potential between said junctions for adjusting one of said control units, and means for reducuhstantially to zero said potential differences among a plurality of final control units comprising a responsive circuit including a plurality oi connected control elements and a including a plurality of series connected adjustable impedance elements, said circuits being connected in parallel to a source potential, detector means connected oetween the junction of two of said control elements and two or": said adjustable impedance elements, means op a le in response to potential differences between sa c us for adjusting one of said control units, and means for reducing substantially to zero said potential differences in response to ustment of said control units.

13. A control system for dividing a manipulated variable among a plurality of final control units comprising a plurality of series connected control elements and a plurality of series connccted adjsutahle impedance elements connected in parallel to a source of potential, one of said control elements and one of said adjustable impedance elements corresponding to each of said plurality of control units, a detector means connected between the junctions of two of said control elements and the corresponding two or said adjustable impedance elements, means operable by said detector means in response to potential differences between said junctions for adjusting one or said control units corresponding to said elements adjacent one of said junctions to which said detector means is connected, and means 0perable in response to adjustment of said control units for reducing said potential differences between said junctions substantially to zero.

14. In a control system, the combination of a plurality of control units, each adjustable to divide a common output, master control means ineluding a totalizing circuit for simultaneously adjusting each of said units to maintain said common output, control elements associated with each of said control units, said elements being connected in series in a responsive circuit to develop potential differences of magnitude related to the fraction of said common output on a corresponding control unit, a reference circuit, adjustable elements connected in series in said refer nce circuit corresponding to each of said units developing potential differences in said reference circuit corresponding to desired fractions of common output, the sum of the potential dii ences across the adjustable elements said i erence circuit being equal to the sum of the potential differences across said respenslve circuit, and means including a detector responsive to potential differences between adadjustable elements and adjacent control elements for adjusting a corresponding control unit and its associated control elements to maintain at suhstantially zero said potential differences between said adjacent elements.

15. A control system in accordance with claim 14 which includes transfer means for manual contrcl of at least one of said control units, said means operating simultaneously to disconnect the corresponding detector from said control system.

16. A system in accordance with claim 14 in which said control elements are potentiahgem crating means and said totalizing circuit is connected in parallel with said series connected control elements.

17. A control system in accordance with claim 1-6 in which said potential-generating means are a plurality of series connected transformer windings upon which there is impressed an alternating current potential corresponding in magnitude with the ir ction or" the common output on each of the corresponding control units.

18. In a control system comprising a plurality of control units each movable between predetermined limits, means for applying a total potential to a circuit comprising a series connected impedance element for each of said control units adjustable in accordance with the position of that unit between said limits, means for applying said total potential to a reference circuit comprising series connected impedance means for each of said units adjustable in accordance with desired relative positions of said units, and means including detectors responsive to the potential difference between each point intermediate between two impedances in said first circuit and a corresponding point intermediate between two impedances in said reference circuit for read- 21 justing the positions of all except one of said units.

19. A control system for a group of final control units comprising control elements corresponding to said units each connected as a parallel branch of a responsive circuit to divide a total current in relation to the adjustments of said units, a reference circuit having corresponding parallel branches for dividing in a predetermined manner a current of the same magnitude as divided by said responsive circuit, detector means responsive to differences between the currents in said branches of said responsive circuit as compared with the currents in associated branches of said reference circuit to adjust said control units to the desired predetermined relationship as predetermined by the current division in said reference branch.

20. A control system for a group of final control units comprising means for establishing a signal related to the deviation of a controlled variable from a predetermined set point, balanceable means operable jointly in response to said signal and the adjustment of said final control units for adjusting said final control units to change the manipulated variable, supplementary control means comprising control elements corresponding to said final control units and adjusted relative to the adjustments of said control units, each of said control elements being connected as a circuit branch of a responsive circuit to divide the total electrical current flowing in said branches in relation to the adjustments of said units, a reference circuit having corresponding parallel branches for dividing in a predetermined manner a total current equal in magnitude to the total current in said responsive circuit, means operable to readjust said final control units in response to current differences until the division of current in said branches of said responsive circuit is the same as the division of current in said branches of said reference circuit.

EDWARD S. BRISTOL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,745,094 Wild et al. Jan. 28, 1930 2,238,431 Haines Apr. 15, 1941 2,498,101 Wannamaker Feb. 21, 1950 

